Redox flow battery

ABSTRACT

A redox flow battery includes a battery cell; a positive electrolyte tank and a negative electrolyte tank configured to store therein a positive electrolyte and a negative electrolyte, respectively; a positive electrolyte circulation path and a negative electrolyte circulation path each configured to allow a corresponding one of the electrolytes to circulate between a corresponding one of the tanks and the battery cell; and a communicating tube including a tube immersed at one open end thereof in the positive electrolyte, stretched at an intermediate portion thereof above levels of both the electrolytes, and immersed at the other open end thereof in the negative electrolyte.

TECHNICAL FIELD

The present invention relates to a redox flow battery.

BACKGROUND ART

As a large-capacity storage battery, a redox flow battery (which mayhereinafter be referred to as “RF battery”) is known, which performscharge and discharge by circulating positive and negative electrolytesthrough a battery cell (see, e.g., Patent Literatures 1 and 2). The RFbattery includes the battery cell, a positive electrolyte tank and anegative electrolyte tank configured to store therein a positiveelectrolyte and a negative electrolyte, respectively, and a positiveelectrolyte circulation path and a negative electrolyte circulation patheach configured to allow a corresponding one of the electrolytes tocirculate between a corresponding one of the tanks and the battery cell.Patent Literatures 1 and 2 each describe an RF battery that includes acommunicating tube configured to allow communication between thepositive electrolyte tank and the negative electrolyte tank.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2013-25964

PTL 2: Japanese Unexamined Patent Application Publication No. 2013-37814

SUMMARY OF INVENTION

A redox flow battery of the present disclosure includes a battery cell;a positive electrolyte tank and a negative electrolyte tank configuredto store therein a positive electrolyte and a negative electrolyte,respectively; a positive electrolyte circulation path and a negativeelectrolyte circulation path each configured to allow a correspondingone of the electrolytes to circulate between a corresponding one of thetanks and the battery cell; and a communicating tube including a tubeimmersed at one open end thereof in the positive electrolyte, stretchedat an intermediate portion thereof above levels of both theelectrolytes, and immersed at the other open end thereof in the negativeelectrolyte.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a redox flow battery according to anembodiment.

FIG. 2 is a schematic diagram of a cell stack.

FIG. 3 illustrates dimensions of a communicating tube included in theredox flow battery according to the embodiment.

FIG. 4 illustrates a modified open end of the communicating tubeincluded in the redox flow battery according to the embodiment.

FIG. 5 is a schematic diagram of a redox flow battery according to afirst modification.

FIG. 6 is a schematic diagram of a redox flow battery according to asecond modification.

DESCRIPTION OF EMBODIMENTS Problems to be Solved by the PresentDisclosure

When charge and discharge cycles are repeated during operation of aredox flow battery (RF battery), a phenomenon called “electrolytecrossover” may occur, in which positive and negative electrolytes areeach transferred from one side to the other through a membraneinterposed between a positive electrode and a negative electrode insidea battery cell. This may create a difference between the volumes of theelectrolytes in a positive electrolyte tank and a negative electrolytetank and lead to reduced battery capacity (discharge capacity).

As a solution to such problems associated with the electrolytecrossover, Patent Literatures 1 and 2 each disclose a technique in whichthe positive electrolyte tank and the negative electrolyte tank areconnected by a communicating tube and when a difference occurs betweenthe volumes of the electrolytes in the tanks, the levels of theelectrolytes in the tanks are adjusted to the same height using thecommunicating tube. In the techniques disclosed in Patent Literatures 1and 2, however, the communicating tube is connected and positioned at alevel substantially the same as, or lower than, the levels of theelectrolytes in the tanks. This may cause electrolyte to leak through aconnection of each tank with the communicating tube, or may causeelectrolyte to flow out of the communicating tube in the event ofbreakage of the communicating tube.

Accordingly, the present disclosure aims to provide a redox flow batterythat is capable not only of adjusting the volumes of the electrolytes inthe positive electrolyte tank and the negative electrolyte tank, butalso of preventing each electrolyte from flowing out of the tank.

Advantageous Effects of the Present Disclosure

The present disclosure provides a redox flow battery that is capable notonly of adjusting the volumes of the electrolytes in the positiveelectrolyte tank and the negative electrolyte tank, but also ofpreventing each electrolyte from flowing out of the tank.

Description of Embodiment of the Invention of the Present Application

First, aspects of an embodiment of the invention of the presentapplication will be listed.

(1) A redox flow battery according to an aspect of the invention of thepresent application includes a battery cell; a positive electrolyte tankand a negative electrolyte tank configured to store therein a positiveelectrolyte and a negative electrolyte, respectively; a positiveelectrolyte circulation path and a negative electrolyte circulation patheach configured to allow a corresponding one of the electrolytes tocirculate between a corresponding one of the tanks and the battery cell;and a communicating tube including a tube immersed at one open endthereof in the positive electrolyte, stretched at an intermediateportion thereof above levels of both the electrolytes, and immersed atthe other open end thereof in the negative electrolyte.

In the redox flow battery described above, the one and other open endsof the communicating tube are immersed in the positive electrolyte andthe negative electrolyte, and the intermediate portion of thecommunicating tube is stretched above the levels of both theelectrolytes. The communicating tube becomes a siphon by being filledwith electrolyte. Then, when a difference occurs between the volumes ofthe electrolytes in the tanks, the levels of the electrolytes areadjusted to the same height through the communicating tube using thesiphon principle. The volumes of the electrolytes are thus automaticallyadjusted to maintain the levels of the electrolytes in the tanks. If thecommunicating tube is broken, the resulting entry of air into thecommunicating tube terminates the siphon. As described above, theintermediate portion of the communicating tube is disposed above thelevels of both the electrolytes. Therefore, even in the event ofbreakage, the electrolyte in the communicating tube is returned toeither of the tanks by termination of the siphon. That is, even if thecommunicating tube is broken, the electrolyte in the communicating tubeis prevented from flowing out of the tank. The redox flow batterydescribed above is thus capable not only of adjusting the volumes of theelectrolytes in the positive electrolyte tank and the negativeelectrolyte tank, but also of preventing each electrolyte from flowingout of the tank.

(2) Another aspect of the redox flow battery may include an introducingtube configured to connect at least one of the positive electrolytecirculation path and the negative electrolyte circulation path to thecommunicating tube, and an open-close valve configured to open and closethe introducing tube.

To serve as a siphon, the communicating tube needs to be filled withelectrolyte. In this aspect of the redox flow battery, which includesthe introducing tube, the electrolyte is introduced through theintroducing tube into the communicating tube when the redox flow batterystarts, and this allows the communicating tube to be filled with theelectrolyte. Also, with the open-close valve, a siphon can be createdwhen the open-close valve closes the introducing tube to block the flowbetween the circulation path and the communicating tube, with thecommunicating tube being filled with the electrolyte.

(3) In another aspect of the redox flow battery, the open ends of thecommunicating tube may each be located on a bottom side in acorresponding one of the tanks.

Entry of a gas into the communicating tube may terminate the siphon. Gasbubbles may be produced in the vicinity of the surface of theelectrolyte in each tank. Therefore, if an open end of the communicatingtube is located near the surface of the electrolyte in the tank, gasbubbles are easily drawn in through the open end. In this aspect of theredox flow battery, where each open end of the communicating tube islocated on the bottom side in the tank, gas bubbles are not easily drawnin through the open end. This makes it easier to keep the communicatingtube in a siphon state.

(4) In another aspect of the redox flow battery, the open ends of thecommunicating tube may be formed to face upward.

In this aspect of the redox flow battery, where the open ends of thecommunicating tube are formed to face upward, gas bubbles are not easilydrawn in through the open ends. This also makes it easier to keep thecommunicating tube in a siphon state.

(5) In another aspect of the redox flow battery, the communicating tubemay be provided with a flow control valve.

In this aspect of the redox flow battery, where the communicating tubeis provided with a flow control valve, it is possible to control, withthe flow control valve, the flow rate (or the amount of transfer) of theelectrolyte passing through the communicating tube when the volumes ofthe electrolytes in the tanks are adjusted through the communicatingtube using the siphon principle.

(6) Another aspect of the redox flow battery may include a gas vent pipeconfigured to allow gas bubbles to escape from inside the communicatingtube, and the gas vent pipe may be connected at one end thereof to thecommunicating tube and connected at the other end thereof to at leastone of the positive electrolyte circulation path and the negativeelectrolyte circulation path.

In this aspect of the redox flow battery, which includes the gas ventpipe, gas bubbles accumulated in the communicating tube can bedischarged through the gas vent pipe to the circulation path. This makesit possible to keep the communicating tube in a siphon state.

Details of Embodiment of the Invention of the Present Application

Examples of a redox flow battery (RF battery) according to an embodimentof the invention of the present application will now be described withreference to the drawings. The same reference numerals in the drawingsdenote the same or corresponding parts. The invention of the presentapplication is not limited to the examples described below, and isdefined by the appended claims. All changes that fall within meaningsand scopes equivalent to the claims are therefore intended to beembraced by the claims.

RF Battery

An RF battery according to the embodiment is typically connected throughan AC/DC converter to an electric system and performs charge anddischarge. The charge and discharge process involves using a positiveelectrolyte and a negative electrolyte each containing, as activematerials, metal ions whose valences are changed by oxidation-reduction.The charge and discharge process is performed using a difference betweenthe oxidation-reduction potential of the ions contained in the positiveelectrolyte and the oxidation-reduction potential of the ions containedin the negative electrolyte.

An RF battery 1 according to the embodiment will now be described withreference to FIGS. 1 and 2. As illustrated in FIG. 1, the RF battery 1of the embodiment includes a battery cell 100, a positive electrolytetank 106 and a negative electrolyte tank 107, and a positive electrolytecirculation path 120 and a negative electrolyte circulation path 130. Afeature of the RF battery 1 is that it includes a communicating tube 40for an adjustment which is made using the siphon principle such thatelectrolytes 10P and 10N stored in the positive electrolyte tank 106 andthe negative electrolyte tank 107, respectively, have the same level.Hereinafter, the configuration of the RF battery 1 will be described indetail.

Battery Cell

As illustrated in FIG. 1, the battery cell 100 includes a positiveelectrode 104, a negative electrode 105, and a membrane 101 interposedbetween the electrodes 104 and 105, and a positive electrode cell 102and a negative electrode cell 103 are formed with the membrane 101therebetween. The membrane 101 is, for example, an ion-exchange membranethat allows hydrogen ions to pass therethrough. The battery cell 100(i.e., the positive electrode cell 102 and the negative electrode cell103) has the positive electrolyte circulation path 120 and the negativeelectrolyte circulation path 130 connected thereto, and allows thepositive electrolyte 10P and the negative electrolyte 10N to circulatetherethrough. The electrolytes 10P and 10N may be ones that contain, asactive materials, metal ions of the same type. For example, theelectrolytes 10P and 10N may each be an electrolyte containing vanadiumions, an electrolyte containing either manganese ions or titanium ions,or an electrolyte containing both manganese ions and titanium ions.

The battery cell 100 may be configured either as a single cell includingone battery cell 100, or as a multicell including a plurality of batterycells 100. In the case of a multicell, a structure called a cell stack 2(see FIG. 2) is used, which is formed by stacking a plurality of batterycells 100. The cell stack 2 includes a substack 200 sandwiched on bothsides by two end plates 220, which are fastened with a fasteningmechanism 230 (see the lower part of FIG. 2). FIG. 2 illustrates astructure including a plurality of substacks 200. The substacks 200 areeach formed by sequentially stacking a cell frame 3, the positiveelectrode 104, the membrane 101, and the negative electrode 105 inlayers (see the upper part of FIG. 2) and sandwiching the resultinglayered body between supply/drainage plates 210 on both sides. Thenumber of battery cells 100 stacked in layers to form the cell stack 2can be appropriately determined.

As illustrated in the upper part of FIG. 2, each cell frame 3 includes abipolar plate 31 disposed between the positive electrode 104 and thenegative electrode 105, and a frame body 32 disposed around the bipolarplate 31. The positive electrode 104 is disposed on one side of thebipolar plate 31, and the negative electrode 105 is disposed on theother side of the bipolar plate 31. The bipolar plate 31 is disposedinside the frame body 32, and a recessed portion 32 o is defined by thebipolar plate 31 and the frame body 32. The recessed portion 32 o isprovided on each side of the bipolar plate 31. The positive electrode104 and the negative electrode 105 are housed in the respective recessedportions 32 o, with the bipolar plate 31 interposed therebetween, and afirst side of the frame body 32 of each cell frame 3 and a second sideof the frame body 32 of an adjacent cell frame 3 are joined oppositeeach other. In the substack 200 (cell stack 2), the positive electrode104 and the negative electrode 105, with the membrane 101 therebetween,are arranged between the bipolar plates 31 of adjacent cell frames 3 toform one battery cell 100. To prevent leakage of electrolyte, the framebodies 32 of adjacent cell frames 3 are each provided with an annularsealing member 37, such as an O ring or flat gasket, therebetween.

For example, the bipolar plate 31 is made of plastic carbon, and theframe body 32 is made of plastic, such as vinyl chloride resin (PVC),polypropylene, polyethylene, fluorine resin, or epoxy resin. In thisexample, each cell frame 3 includes the bipolar plate 31 and the framebody 32 therearound that are integrally formed, for example, byinjection molding.

The circulation of electrolyte into the battery cell 100 is made throughthe supply/drainage plates 210 (see the lower part of FIG. 2) and alsothrough liquid supply manifolds 33 and 34 and liquid discharge manifolds35 and 36 passing through the frame body 32 of each cell frame 3 (seethe upper part of FIG. 2) and liquid supply slits 33 s and 34 s andliquid discharge slits 35 s and 36 s formed in the frame body 32. In thecase of the cell frame 3 (frame body 32) of this example, the positiveelectrolyte is supplied from the liquid supply manifold 33 in the lowerpart of the frame body 32, through the liquid supply slit 33 s on thefirst side of the frame body 32, to the positive electrode 104, and thendischarged through the liquid discharge slit 35 s in the upper part ofthe frame body 32 to the liquid discharge manifold 35. Similarly, thenegative electrolyte is supplied from the liquid supply manifold 34 inthe lower part of the frame body 32, through the liquid supply slit 34 son the second side of the frame body 32, to the negative electrode 105,and then discharged through the liquid discharge slit 36 s in the upperpart of the frame body 32 to the liquid discharge manifold 36. Aflow-guiding portion (not shown) may be formed along the lower and upperinner edges of the frame body 32 having the bipolar plate 31 therein.The flow-guiding portions have the function of diffusing theelectrolytes supplied through the liquid supply slits 33 s and 34 salong the lower edges of the electrodes 104 and 105, and collecting theelectrolytes discharged from the upper edges of the electrodes 104 and105 into the liquid discharge slits 35 s and 36 s.

Positive Electrolyte Tank and Negative Electrolyte Tank

As illustrated in FIG. 1, the positive electrolyte tank 106 and thenegative electrolyte tank 107 store therein the positive electrolyte 10Pand the negative electrolyte 10N, respectively. The tanks 106 and 107are of the same shape and capacity. The upper part of the interior ofeach of the tanks 106 and 107 (i.e., upper part above the level of eachof the electrolytes 10P and 10N) is a gas-phase portion. The positiveelectrolyte tank 106 has an outlet 61 and an inlet 62 connected to asupply pipe 108 and a return pipe 110, respectively, of the positiveelectrolyte circulation path 120. The negative electrolyte tank 107 hasan outlet 71 and the inlet 72 connected to a supply pipe 109 and areturn pipe 111, respectively, of the negative electrolyte circulationpath 130. In this example, the outlets 61 and 71 and the inlets 62 and72 are located above the levels of the electrolytes 10P and 10N in thetanks 106 and 107, or specifically, at the tops of the tanks 106 and107. The outlet 61 and the inlet 62 are each provided with an open-closevalve 66, and the outlet 71 and the inlet 72 are each provided with anopen-close valve 76.

The tanks 106 and 107 have openings 64 and 74, respectively, to whichthe communicating tube 40 is connected. The openings 64 and 74 aredisposed above the levels of the electrolytes 10P and 10N in the tanks106 and 107. In this example, the openings 64 and 74 are located at thetops of the tanks 106 and 107. The openings 64 and 74 are provided withopen-close valves 67 and 77, respectively.

Positive Electrolyte Circulation Path and Negative ElectrolyteCirculation Path

As illustrated in FIG. 1, the positive electrolyte circulation path 120connects the positive electrolyte tank 106 to the battery cell 100,whereas the negative electrolyte circulation path 130 connects thenegative electrolyte tank 107 to the battery cell 100, thereby allowingthe electrolytes 10P and 10N to circulate between the tanks 106 and 107and the battery cell 100. The positive electrolyte circulation path 120includes the supply pipe 108 configured to supply the positiveelectrolyte 10P from the positive electrolyte tank 106 to the positiveelectrode cell 102, and the return pipe 110 configured to return thepositive electrolyte 10P from the positive electrode cell 102 to thepositive electrolyte tank 106. The negative electrolyte circulation path130 includes the supply pipe 109 configured to supply the negativeelectrolyte 10N from the negative electrolyte tank 107 to the negativeelectrode cell 103, and the return pipe 111 configured to return thenegative electrolyte 10N from the negative electrode cell 103 to thenegative electrolyte tank 107. The supply pipes 108 and 109 of thecirculation paths 120 and 130 are connected to the outlets 61 and 71,respectively, of the tanks 106 and 107, and the return pipes 110 and 111of the circulation paths 120 and 130 are connected to the inlets 62 and72, respectively, of the tanks 106 and 107.

End portions of the respective supply pipes 108 and 109 are insertedthrough the outlets 61 and 71, respectively, into the tanks 106 and 107,and open ends 81 and 91 of these end portions are disposed below thelevels of the electrolytes 10P and 10N in the tanks 106 and 107. Thatis, the open ends 81 and 91 of the supply pipes 108 and 109 are immersedin the electrolytes 10P and 10N, respectively, which are drawn inthrough the open ends 81 and 91. In this example, the open ends 81 and91 of the supply pipes 108 and 109 are each located on the bottom sidein the corresponding one of the tanks 106 and 107. Note that “located onthe bottom side in the tank” refers to being located below the level ofthe electrolyte 10P or 10N, that is, h/2 or less from the bottom of thetank 106 or 107, where h is a height from the bottom of the tank 106 or107 to the surface of the electrolyte 10P or 10N.

The supply pipes 108 and 109 are provided with pumps 112 and 113,respectively, configured to suck up the electrolytes 10P and 10N fromthe tanks 106 and 107 and pressure-feed them. During charge anddischarge operation, the pumps 112 and 113 circulate the electrolytes10P and 10N, respectively, through the battery cell 100 (i.e., thepositive electrode cell 102 and the negative electrode cell 103). Instandby mode where neither charge nor discharge takes place, the pumps112 and 113 are off and the electrolytes 10P and 10N are not circulated.

Communicating Tube

As illustrated in FIG. 1, the communicating tube 40 is a tube immersedat one open end 41 thereof in the positive electrolyte 10P, stretched atan intermediate portion 43 thereof above the levels of the electrolytes10P and 10N, and immersed at the other open end 42 thereof in thenegative electrolyte 10N. The communicating tube 40 is configured toallow liquid-phase portions in the tanks 106 and 107 to communicate witheach other. The communicating tube 40 is connected to the openings 64and 74 of the tanks 106 and 107. In this example, the communicating tube40 is inserted at both end portions thereof through the openings 64 and74 of the tanks 106 and 107 into the tanks 106 and 107, and the openends 41 and 42 of both the end portions are disposed below the levels ofthe electrolytes 10P and 10N in the tanks 106 and 107. The intermediateportion 43 is placed above the tanks 106 and 107. In this example, theopen ends 41 and 42 of the communicating tube 40 are each located on thebottom side in a corresponding one of the tanks 106 and 107.

The communicating tube 40 becomes a siphon by being filled withelectrolyte. Thus, when a difference occurs between the volumes of theelectrolytes 10P and 10N in the tanks 106 and 107, the levels of theelectrolytes 10P and 10N are adjusted to the same height using thesiphon principle. When the tube forming the communicating tube 40 (orthe intermediate portion 43 in particular) is made of a transparentmaterial, it is possible to visually recognize from the outside that thecommunicating tube 40 is filled with electrolyte. The intermediateportion 43 may have a window at the top (or highest portion) thereof,and the window may be made of a transparent material. Examples of thetransparent material include transparent resin, such as vinyl chlorideresin, and glass.

The communicating tube 40 may be appropriately designed to satisfy thesiphon principle. Dimensions of the communicating tube will now bedescribed with reference primarily to FIG. 3.

If a height H from the level of the electrolytes 10P and 10N in thetanks 106 and 107 to the top of the communicating tube 40 is too high,the transfer of electrolyte based on the siphon principle fails. Themaximum height H_(max) that satisfies the siphon principle is determinedby the following equation:

H _(max) =P0/ρ·g

where P0 (N/m²) is pressure in the tank, ρ (kg/m³) is an electrolytedensity, and g (m/s²) is the acceleration of gravity.

When the pressure P0 is equal to the atmospheric pressure (1.013×10⁵N/m²) and the electrolyte density ρ is 1400 kg/m³, then H_(max) is 7.38m. Therefore, the installation level of the intermediate portion 43(corresponding to the height H) is less than 7.38 m from the level ofthe electrolytes 10P and 10N in the tanks 106 and 107.

If a length L of the communicating tube 40 is too long, the resultingincrease in frictional resistance in the communicating tube 40 leads toan increased flow friction loss and lowers the flow rate of theelectrolyte passing through the communicating tube 40. This means thatit takes time to adjust the electrolyte levels. The adjustment of theelectrolyte levels is preferably completed within 10 minutes (or 600seconds). Therefore, the length L of the communicating tube 40 ispreferably set to ensure that the flow rate of the electrolyte passingthrough the communicating tube 40 is above a certain level. For example,the length L may be 15 m, or more preferably 10 m or less.

An inside diameter d of the communicating tube 40 may also beappropriately set. For example, the inside diameter d may range from 10mm to 150 mm, or more preferably from 20 mm to 100 mm. The flow frictionloss varies depending also on the inside diameter d of the communicatingtube 40. That is, the smaller the inside diameter d, the greater theflow friction loss. Therefore, it is preferable to appropriately set thelength L of the communicating tube 40 in accordance with the insidediameter d. Specifically, it is preferable to set the length L of thecommunicating tube 40 such that when a difference occurs between thevolumes of the electrolytes 10P and 10N in the tanks 106 and 107, ittakes 10 minutes (or 600 seconds) or less until the levels of theelectrolytes 10P and 10N reach the same height. In this case, forexample, the length L of the communicating tube 40 may be less than orequal to 100 times the inside diameter d (L≤100d). Specifically, thelength L may be 10 m or less if the inside diameter d is 100 mm, L maybe 4.5 m or less if d is 50 mm, and L may be 1.7 m or less if d is 20mm.

Introducing Tube

The RF battery 1 illustrated in FIG. 1 includes an introducing tube 50configured to connect the positive electrolyte circulation path 120 tothe communicating tube 40, and an open-close valve 51 configured to openand close the introducing tube 50. In this example, the introducing tube50 is connected at one end thereof in such a manner as to branch off thesupply pipe 108 of the positive electrolyte circulation path 120 and isconnected at the other end thereof to the intermediate portion 43 of thecommunicating tube 40. Also, in this example, the supply pipe 108 has anopen-close valve 69 downstream of its connection with the introducingtube 50 (i.e., located closer to the battery cell 100 than theconnection is).

The introducing tube 50 and the open-close valve 51 are used to form asiphon, when the RF battery 1 starts, by filling the communicating tube40 with electrolyte. Specifically, when the RF battery 1 starts, theopen-close valve 51 is opened to start the pump 112, with the supplypipe 108 and the communicating tube 40 communicating with each otherthrough the introducing tube 50, and circulate the positive electrolyte10P through the communicating tube 40. This makes it possible tointroduce the positive electrolyte 10P into the communicating tube 40and fill the communicating tube 40 with the electrolyte. Then, with thecommunicating tube 40 being filled with the electrolyte, the open-closevalve 51 closes the introducing tube 50 to block the flow between thesupply pipe 108 and the communicating tube 40. This creates aliquid-tight state in the communicating tube 40 and thereby forms asiphon. After the RF battery 1 starts, the open-close valve 51 is alwaysin a closed state during the operation.

When, for example, the communicating tube 40 is removed from the tanks106 and 107 for maintenance of the RF battery 1, the pump 112 is stoppedand the open-close valve 51 is opened. This terminates the siphon,causes the electrolyte in the communicating tube 40 to return to thetanks 106 and 107, and thereby facilitates the maintenance work. Afterthe communicating tube 40 is removed from the tanks 106 and 107, theopen-close valves 67 and 77 are closed to prevent air from entering thetanks 106 and 107 through the openings 64 and 74. This inhibitsoxidation of the electrolytes 10P and 10N during the maintenance work.

Although the introducing tube 50 is connected and attached to thepositive electrolyte circulation path 120 (supply pipe 108) in thisexample, the introducing tube 50 may be attached to the negativeelectrolyte circulation path 130 (supply pipe 109) or may be attached toboth the circulation paths 120 and 130.

Gas-Phase Communicating Tube

The RF battery 1 illustrated in FIG. 1 includes a gas-phasecommunicating tube 45 that allows the gas-phase portions in the tanks106 and 107 to communicate with each other. With the gas-phasecommunicating tube 45, it is possible to equalize pressures in the tanks106 and 107. The gas-phase communicating tube 45 is stretched over thetanks 106 and 107. In this example, the gas-phase communicating tube 45is connected to openings 65 and 75 at the tops of the tanks 106 and 107.The openings 65 and 75 are provided with open-close valves 68 and 78,respectively.

Advantageous Effects of Embodiment

The RF battery 1 according to the embodiment described above has thefollowing operational advantages.

With the communicating tube 40, when a difference occurs between thevolumes of the electrolytes 10P and 10N in the tanks 106 and 107 duringoperation of the RF battery 1, the levels of the electrolytes 10P and10N can be automatically adjusted to the same level through thecommunicating tube 40 using the siphon principle. As described above,the intermediate portion 43 of the communicating tube 40 is disposedabove the levels of the electrolytes 10P and 10N. Therefore, even if thecommunicating tube 40 is broken, the electrolyte in the communicatingtube 40 is returned to either of the tanks 106 and 107 by termination ofthe siphon. Thus, even in the event of breakage of the communicatingtube 40, it is possible to prevent the electrolyte in the communicatingtube 40 from flowing out of the tanks 106 and 107. Additionally, sincethe openings 64 and 74 of the tanks 106 and 107 to which thecommunicating tube 40 is connected are located above the levels of theelectrolytes 10P and 10N, the electrolytes 10P and 10N are preventedfrom leaking through the openings 64 and 74. Therefore, it is possibleto effectively prevent the electrolytes 10P and 10N from flowing out ofthe positive electrolyte tank 106 and the negative electrolyte tank 107while automatically adjusting the volumes of the electrolytes 10P and10N in the tanks 106 and 107.

With the introducing tube 50 and the open-close valve 51, a siphon canbe created by filling the communicating tube 40 with electrolyte whenthe RF battery 1 starts.

Since the open ends 41 and 42 of the communicating tube 40 are locatedon the bottom side in the tanks 106 and 107, gas bubbles are not easilydrawn in through the open ends 41 and 42. This makes it easier to keepthe communicating tube 40 in a siphon state.

As illustrated in FIG. 4, the communicating tube 40 may be bent into a Jshape at both end portions thereof to allow the open ends 41 and 42 toface upward. In this case, gas bubbles are not easily drawn in throughthe open ends 41 and 42, and this makes it easier to maintain the siphonstate.

Modifications of the RF battery 1 according to the aforementionedembodiment will now be described with reference to FIGS. 5 and 6.

First Modification

The RF battery 1 according to a first modification illustrated in FIG. 5differs from the aforementioned embodiment illustrated in FIG. 1 in thatthe communicating tube 40 is provided with a flow control valve 44.Other configurations of the first modification are the same as those ofthe aforementioned embodiment.

Flow Control Valve

The flow control valve 44 is disposed in the communicating tube 40 andconfigured to control the flow rate of the electrolyte passing throughthe communicating tube 40. In this example, as illustrated in FIG. 5,the flow control valve 44 is disposed in the intermediate portion 43 ofthe communicating tube 40. When, in the RF battery 1 of the firstmodification, the volumes of the electrolytes 10P and 10N in the tanks106 and 107 are adjusted through the communicating tube 40, the flowrate (or the amount of transfer) of the electrolyte can be controlled bythe flow control valve 44. In some cases, the transfer of theelectrolyte may be stopped by closing the flow control valve 44. In thisexample, the flow control valve 44 is a motor-operated valve.

Second Modification

The RF battery 1 according to a second modification illustrated in FIG.6 differs from the aforementioned embodiment illustrated in FIG. 1 inthat it includes a gas vent pipe 55 for allowing gas bubbles to escapefrom inside the communicating tube 40. Other configurations of thesecond modification are the same as those of the aforementionedembodiment.

Gas Vent Pipe

The gas vent pipe 55 is used to vent, from the communicating tube 40,gas bubbles accidentally drawn into the communicating tube 40. The gasvent pipe 55 is connected at one end thereof to the communicating tube40 and connected at the other end thereof to at least one of thepositive electrolyte circulation path 120 and the negative electrolytecirculation path 130. In this example, as illustrated in FIG. 6, the gasvent pipe 55 is connected at one end thereof to the intermediate portion43 in such a manner as to branch off the communicating tube 40 andconnected at the other end thereof to the supply pipe 109 of thenegative electrolyte circulation path 130 in such a manner as to jointhe supply pipe 109. More specifically, the gas vent pipe 55 isconnected at one end thereof to the top of the intermediate portion 43and connected at the other end thereof to the supply pipe 109 at alocation upstream of the pump 113 in the supply pipe 109 (i.e., closerto the tank 107 than the pump 113 is).

The gas vent pipe 55 is provided with a check valve 56. The check valve56 is disposed in the gas vent pipe 55 and configured to block thecirculation from the negative electrolyte circulation path 130 (supplypipe 109) to the communicating tube 40. Additionally, in this example,an open-close valve 57 is provided downstream of the check valve 56(i.e., closer to the supply pipe 109 than the check valve 56 is). Theopen-close valve 57 is in an open state when gas bubbles are to beremoved from the communicating tube 40, and is in a closed state whenthere is no need to remove gas bubbles from the communicating tube 40.

In the RF battery 1 of the second modification, if gas bubbles areaccidentally drawn into the communicating tube 40, the gas bubblesaccumulated in the communicating tube 40 can be vented through the gasvent pipe 55 to the negative electrolyte circulation path 130 (supplypipe 109) using suction by the pump 113. This allows the communicatingtube 40 to be kept in a siphon state. With the gas vent pipe 55 havingthe check valve 56, even when the pump 113 is stopped and the supplypipe 109 is emptied, the entry of gas from the supply pipe 109 into thecommunicating tube 40 can be blocked. Therefore, even when the pump 113is stopped, the communicating tube 40 is kept in a siphon state and theelectrolyte in the communicating tube 40 is not returned into either ofthe tanks 106 and 107.

Additionally, by adjusting the degree of opening of the open-close valve57, it is possible to control the flow rate of the electrolyte passingthrough the gas vent pipe 55 and prevent the electrolyte fromaccidentally flowing out of the communicating tube 40 into the supplypipe 109. Specifically, suction by the pump 113 causes the electrolyteto be fed little by little through the gas vent pipe 55 and thecommunicating tube 40 to the supply pipe 109, so that gas bubblesaccumulated in the communicating tube 40 are efficiently removed. Whenthe communicating tube 40 is removed, for example, for maintenance ofthe RF battery 1, the entry of air into the supply pipe 109 is preventedby closing the open-close valve 57.

Application of Embodiment

The redox flow battery according to the embodiment can be used for loadleveling, compensation for instantaneous voltage drop, emergency powersupply, and smoothing of the output of natural energy-based powergeneration (e.g., solar or wind power generation) which has beenintroduced at a large scale.

REFERENCE SIGNS LIST

1: redox flow battery (RF battery)

2: cell stack

3: cell frame

31: bipolar plate

32: frame body

32 o: recessed portion

33, 34: liquid supply manifold

35, 36: liquid discharge manifold

33 s, 34 s: liquid supply slit

35 s, 36 s: liquid discharge slit

37: sealing member

40: communicating tube

41, 42: open end

43: intermediate portion

44: flow control valve

45: gas-phase communicating tube

50: introducing tube

51: open-close valve

55: gas vent pipe

56: check valve

57: open-close valve

61, 71: outlet

62, 72: inlet

64, 74: opening

65, 75: opening

66, 67, 68, 69, 76, 77, 78: open-close valve

81, 91: open end

100: battery cell

101: membrane

102: positive electrode cell

103: negative electrode cell

104: positive electrode

105: negative electrode

106: positive electrolyte tank

107: negative electrolyte tank

108, 109: supply pipe

110, 111: return pipe

112, 113: pump

120: positive electrolyte circulation path

130: negative electrolyte circulation path

10P: positive electrolyte

10N: negative electrolyte

200: substack

210: supply/drainage plate

220: end plate

230: fastening mechanism

1. A redox flow battery comprising: a battery cell; a positiveelectrolyte tank and a negative electrolyte tank; a positive electrolytecirculation path and a negative electrolyte circulation path eachconfigured to allow an electrolyte to circulate between a correspondingone of the tanks and the battery cell; and a communicating tubeconfigured to allow an interior of the positive electrolyte tank tocommunicate with an interior of the negative electrolyte tank, whereinthe communicating tube has one open end located in the interior of thepositive electrolyte tank and the other open end located in the interiorof the negative electrolyte tank; a height of an intermediate portion ofthe communicating tube is higher than a height of the one open end, theintermediate portion being disposed between the one open end and theother open end; and the height of the intermediate portion of thecommunicating tube is higher than a height of the other open end.
 2. Theredox flow battery according to claim 1, wherein the positiveelectrolyte tank and the negative electrolyte tank each have a top, abottom, and a side; and the communicating tube has a first extendingportion extending from the one open end to the top of the positiveelectrolyte tank, the intermediate portion located in an outside regionbetween the positive electrolyte tank and the negative electrolyte tank,and a second extending portion extending from the top of the negativeelectrolyte tank to the other open end.
 3. The redox flow batteryaccording to claim 2, wherein the first extending portion has a firstend corresponding to the one open end of the communicating tube; adistance from the top of the positive electrolyte tank to the first endis greater than a distance from the bottom of the positive electrolytetank to the first end; the second extending portion has a second endcorresponding to the other open end of the communicating tube; and adistance from the top of the negative electrolyte tank to the second endis greater than a distance from the bottom of the negative electrolytetank to the second end.
 4. The redox flow battery according to claim 3,further comprising: a positive electrolyte stored in the positiveelectrolyte tank; and a negative electrolyte stored in the negativeelectrolyte tank, wherein the first end is located below a level of thepositive electrolyte stored in the positive electrolyte tank; the firstend is located at a distance of h/2 or less from the bottom of thepositive electrolyte tank, where h is a height from the bottom of thepositive electrolyte tank to the level of the positive electrolyte; thesecond end is located below a level of the negative electrolyte storedin the negative electrolyte tank; and the second end is located at adistance of H/2 or less from the bottom of the negative electrolytetank, where H is a height from the bottom of the negative electrolytetank to the level of the negative electrolyte.
 5. The redox flow batteryaccording to claim 2, wherein the first extending portion extendssubstantially in a height direction; and the second extending portionextends substantially in the height direction.
 6. The redox flow batteryaccording to claim 1, wherein at least a portion of the communicatingtube is made of a transparent material to make an interior of thecommunicating tube visible from outside.
 7. The redox flow batteryaccording to claim 1, wherein the communicating tube has a window in ahighest region thereof, and the window is made of a transparentmaterial.
 8. The redox flow battery according to claim 1, wherein alength L from the one open end to the other open end of thecommunicating tube is 10 m or less.
 9. The redox flow battery accordingto claim 1, wherein the communicating tube has an inside diameter d of10 mm or more and 150 mm or less.
 10. The redox flow battery accordingto claim 1, wherein a length L from the one open end to the other openend of the communicating tube is 10 m or less; an inside diameter d ofthe communicating tube is 10 mm or more and 150 mm or less; and thelength L is less than or equal to 100 times the inside diameter d. 11.The redox flow battery according to claim 1, further comprising a gasvent pipe configured to allow air bubbles to escape from inside thecommunicating tube, wherein the gas vent pipe is connected at one endthereof to the communicating tube and connected at the other end thereofto at least one of the positive electrolyte circulation path and thenegative electrolyte circulation path.
 12. The redox flow batteryaccording to claim 1, further comprising: a positive electrolyte storedin the positive electrolyte tank; and a negative electrolyte stored inthe negative electrolyte tank, wherein the communicating tube isimmersed at both ends thereof in the respective electrolytes, and aninterior of the communicating tube is filled with the positiveelectrolyte and the negative electrolyte.
 13. The redox flow batteryaccording to claim 12, wherein the height of the intermediate portion ofthe communicating tube is higher than a level of the positiveelectrolyte stored in the positive electrolyte tank and a level of thenegative electrolyte stored in the negative electrolyte tank.
 14. Theredox flow battery according to claim 12, wherein at least a portion ofthe communicating tube is made of a transparent material to make aninterior of the communicating tube visible from outside.
 15. The redoxflow battery according to claim 12, wherein the communicating tube has awindow in a highest region thereof, and the window is made of atransparent material.